The present invention relates generally to a method of concentrating iron ore and, more particularly, to a method of upgrading fine-grained earthy hematite iron ores.
In recent years, the percentage of steel being produced throughout the world using electric arc furnaces has increased to approximately 30%. However, unlike some other steel manufacturing systems, electric arc furnaces require relatively high grade ore in which the total iron concentration is greater than 68% with an oxide gangue concentration of less than 3%. Materials with oxide gangue concentrations much beyond this level produce an excessive volume of slag and therefore are uneconomically feasible as charge material.
Numerous relatively high-grade iron ores with iron concentrations in the range of 60 to 65% are of an earthy nature. These ores, which proliferate in Western Australia, the Middle East, and Africa, are typically intimately associated with extremely fine grained silica and alumina of feldspathic origin. Unfortunately, due to the fine grain structure associated with these ores, liberating the iron and achieving the oxide contaminant levels required for electric arc furnace charge material is nearly impossible and is generally cost prohibitive.
Several different processes have been developed for recovering iron concentrates from ore. U.S. Pat. No. 2,944,884 discloses a technique for producing high iron concentrates from low grade ores such as taconite. In the disclosed technique, the low grade ore is first crushed to minus xe2x85x9c inch or finer. The crushed ore is then mixed with a reactive form of carbon such that the mixture contains at least 50% more carbon than the theoretical quantity needed for complete reduction. The mixture is then heated for a period of time between 18 and 21 hours at a temperature of about 870 to 1100xc2x0 C. so that the iron becomes fully reduced and carburized. The material is rapidly cooled so that the iron carbide particles lose their malleability. The charge is then ground and the carburized iron particles containing at least about 0.65% carbon are magnetically separated from the gangue particles.
U.S. Pat. No. 2,986,460 discloses a direct reduction process in which the iron ore is mixed with a carbonaceous reducing agent and then heated in a rotary kiln at temperatures of about 900xc2x0 C. The material is then cooled under controlled conditions in a non-oxidizing environment. The reduced iron is then separated from most of the gangue and compacted into briquettes.
In a more recent advance, U.S. Pat. No. 4,416,688 discloses a technique for reducing high phosphorus iron ore. In the disclosed technique sponge iron produced by selective solid state reduction is ground using a ball mill. The hammering action of the ball mill causes the formation of iron flakes of approximately 0.01 to 0.1 millimeters in size along with finely divided oxide gangue. Conventional concentration techniques are used to obtain iron flake powder concentrate. The carbon content of the sponge iron must be kept to a minimum, preferably below 0.10% and at least below 0.25%. Besides placing restrictions on the carbon content, the ability of the disclosed process to obtain high iron concentrates with low oxide contents is hampered by the size of the iron flakes formed by the ball mill. As a result, the efficiency and therefore the cost effectiveness of this approach is lower than desirable.
Therefore a cost effective technique for obtaining super concentrate materials with high iron concentrations and low oxide concentrations from relatively rich, fine-grained iron ores is needed.
The present invention provides a method of upgrading relatively rich, fine-grained earthy hematite iron ores. The iron ore, after suitable preparation, is reduced using any of a variety of direct reduction techniques. For example, the ore may be subjected to a high temperature reduction utilizing hydrogen as the reductant gas. As a result of the reduction process, the iron grains undergo size enhancement. Under the same conditions the contaminant oxides of silica, alumina, apatite, lime, and magnesia are calcined, but unreduced, and remain as refractory oxide gangue.
After completion of the reduction process, the enlarged malleable metallic iron grains are crushed in such a way as to cause the iron grains to fuse together, forming large, flat iron flakes. In order to achieve maximum flake size, the crushing system applies a relatively gradual pressing force rather than a rapid, impact type of force. In one embodiment of the invention, a roll crusher is used to create flakes greater than 1.0 millimeter in size, and typically in the range of 3.0 to 5.0 millimeters in size. As the large flakes are formed, the iron grains are liberated from the refractory oxide grains resulting in an increase in density from about 4 to 5 grams per cubic centimeter to about 6 to 7 grams per cubic centimeter. At the same time that the mechanical working of the reduced iron by the crushing means causes the fusion of the iron grains into flat flakes, the fineness of the residual oxide grains increases.
The shape, size, density, and ferromagnetic differences between the iron flakes and the nonmetallic oxides facilitate separation of the iron. A variety of different separation techniques may be used, including screens, jigs, spirals, elutriation, cyclones, magnetic, and gravity separation. Final concentration and cleaning can be accomplished by low-intensity magnetic separation of the ferromagnetic metal flakes from the nonmagnetic residual oxides.
In one embodiment of the invention, the crushed, reduced material is separated using a mesh screen. The large particles, preferably those particles having a size greater than 3.0 millimeters, undergo flash grinding to further liberate the iron from the nonmetallic oxides. The small particles, those passing through the screen, undergo a longer period of grinding, in the range of 20 to 90 minutes. The iron concentrate is separated from the oxides using a low intensity magnetic separator.
The combination of solid state reduction, mechanical working, and physical/electromagnetic separation enable consistent production of super concentrates of material with metallic iron contents exceeding 92% with less than 5% oxide gangue and an iron recovery of greater than 95%. If optimized, this method can achieve an iron content of between 94% and 98% with less than 2% oxide gangue.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.